Si la gráfica de coseno es negativa tiene forma de montaña?

What is the missing step in this proof ?

1) in triangle the, what is the measure of angle t (in degrees)? 2) in triangle bat, the measure of angle b is 66∘, and the measure of angle t is 77∘.what is the measure of the exterior angle at a (in degrees)?

Having a hard time understanding this question (see attachment).